1. Field of the Invention
The invention relates generally to a hydrostatic drive system for a vehicle, such as an industrial truck having a hydrostatic traction drive system, a hydraulic work system and a hydraulic steering system.
2. Description of the Prior Art
On known hydrostatic drive systems for a vehicle, such as for a fork-lift or a wheel loader, the hydrostatic traction drive system generally has an adjustable-delivery pump and at least one traction drive motor connected to the pump in a closed circuit. To provide power to the hydraulic work system, there is an additional pump with a constant delivery volume which is operated in an open circuit and sucks hydraulic fluid from a reservoir and, when the hydraulic work system is not in operation, returns the hydraulic fluid to the reservoir. The steering system is supplied with hydraulic fluid by an additional pump that also has a constant delivery volume and is operated in an open circuit. This pump can also be used to supply control functions, for example, for a parking brake or a control device that pressurizes the adjustment device of the drive system pump.
On such drive systems, therefore, a plurality of pumps, for example three, are required to supply the traction drive system, the steering system and the hydraulic work system with hydraulic fluid. The drive system therefore takes up a great deal of space. Additionally, all the pumps are continuously driven by a drive motor, although as a rule only one or two pumps are used simultaneously during the operation of the vehicle. This results in corresponding idle losses which in turn result in a low efficiency for the drive system. The continuous operation of all the pumps also results in unnecessary wear.
There are also problems which occur with a traction drive system operated in a closed circuit. For example, during the operation of the traction drive system, leaks occur at the pump and the traction drive motor from the high-pressure side to the low-pressure side of the hydraulic circuit. As a result of conventional measures adopted on the pump to reduce pulsations, the leakage of the pump generally exceeds the leakage at the traction drive motor.
If the traction drive system is in traction operation and if the pump is being operated at a specified setting of the delivery volume and speed, there is a volume current that, at a specified intake volume setting of the traction drive motor, corresponds to a setpoint speed of the traction drive motor and thus to a setpoint speed of the vehicle. As a result of the leakage at the pump and the traction drive motor, however, only the volume current of the pump minus the leakage of the pump and the traction drive motor can be converted into a speed of the traction drive motor. The speed on the traction drive motor is therefore lower than the setpoint speed by the magnitude of the leakage. In traction operation, therefore, the actual speed of travel of the vehicle is generally lower than the setpoint speed of the vehicle set at the pump due to the leakage that occurs at the pump and the traction drive motor.
If the setting of the pump and of the traction drive motor is unchanged and the vehicle travels down a steep slope, the traction drive system switches over into a coasting operation. The traction drive motor begins to function as a pump and the pump begins to function as a motor. As a result, the high-pressure side and the low-pressure side of the hydraulic circuit are reversed and the traction drive motor delivers hydraulic fluid to the pump. The speed set at the traction drive motor is thereby a speed that is higher than the setpoint speed of the traction drive motor set at the pump by the leakage of the pump and the traction drive motor. The speed of travel of the vehicle is therefore a speed that is higher than the speed set at the pump by the magnitude of the leakage that occurs at the traction drive motor and the pump. Therefore, when the vehicle switches from traction operation to coasting operation, there is an increase in the speed of the vehicle that corresponds to twice the leakage on the traction drive motor and the pump. Because the leakage of the pump is thereby greater than the leakage of the traction drive motor, the speed increase of the vehicle when it switches from traction to coasting is essentially determined by the leakage of the pump.
The increase in speed when a vehicle equipped with a closed traction drive system switches from traction to coasting operation is even greater because the pump that is functioning as a motor is driven by the traction drive motor that is functioning as a pump. Therefore, energy is fed into the output side of the drive motor. The speed of the drive motor is thereby increased, as a result of which the pump functioning as a motor is operated at a higher speed. Thus, the traction drive motor can also be operated at a higher speed. The speed of the vehicle when it switches from traction to coasting operation therefore also increases as a result of the speeding up of the drive motor.
The increase in the speed of the drive motor also results in an increase in the level of noise and vibrations generated. Because the braking action continues to be governed by the braking moment that can be absorbed by the drive motor, the braking effect can be too great when the vehicle is empty. On the other hand, when the vehicle is fully loaded, the braking moment of the drive motor may not be sufficient to decelerate the vehicle, as a result of which the braking action is poor and the drive motor is speeded up to undesirably high speeds.
Therefore, it is an object of the invention to provide a drive system that is compact and in which the switchover from traction to coasting operation is improved.